1. Field of the Invention
The invention is directed to a light tap for use in an optical communications system, and more particularly, to a light tap designed for feedback stabilization of laser light sources.
2. Description of the Prior Art
In an optical communications system it is important that the light sources, illustratively semiconductor junction lasers, provide a constant amplitude of light into the system throughout their lifetimes. However, the light output of junction lasers varies with age, temperature, and drive current. The prior art has attempted to solve this problem by the following light monitoring and feedback arrangement wherein emission from both the front and back mirror faces of a junction laser is utilized. Light emitted from the front mirror is coupled into an optical fiber which transmits the signal through the system. Light emitted from the back mirror is converted to an electrical signal which is coupled to feedback circuitry which varies the laser drive current in response to variations in intensity of light emitted from the back mirror. There are several problems with back-mirror monitoring. First, the intensity of the back-mirror emission which is detected for feedback purposes may not be a constant proportion of the front-mirror emission which is coupled into the system. Second, the back-mirror emission is insensitive to variations in the amount of light coupled from the front mirror into the optical fiber. These variations can result from a number of sources, e.g., shifting of the lasing filament (beam wander) or of the position of the fiber with respect to the laser active region.
In a communications system it is the amplitude of light which enters the optical fiber and is subsequently propagated in the system which must be maintained substantially constant. Even assuming that a laser could be made whose light output was time invariant, aging of the package which structurally supports the source and optical fiber can cause axial and lateral displacement of the fiber with respect to the laser, thus, causing variations in the amount of light entering the fiber and in the modal distribution of the propagated light. Therefore, even with an ideal source it is important to monitor the light propagating in the fiber.
One technique for monitoring the light propagating through the fiber is to use a light tap to extract a portion of the propagating light. The extracted portion of the light can then be converted to an electrical signal which is employed in a feedback scheme for controlling the laser drive current. In order for the tap monitoring system to be effective, the extracted portion of the light should be a substantially constant proportion of the amount of light propagating through the fiber. However, as discussed above, laser beam wander and laser-to-fiber displacement as a result of aging cause time variations in the intensity and modal distribution of the propagating light. Unfortunately, prior art taps, such as those constructed with tapered fibers or those which expose the fiber core to another material of higher index of refraction, are highly mode sensitive because they sample the higher order modes propagating furthest from the longitudinal axis of the fiber. Thus, the ratio of the intensity of the extracted tap light to the intensity of the propagating light will vary with time as the modal distribution changes.